In the design of Low Earth Orbital (hereafter LEO) constellation, a careful compromise must be done between satellite capabilities (e.g. power consumption and available memory), ground segment (e.g. number of ground stations and feeder link capacity) and service requirement (e.g. network capacity and processing delay).
Usually four main solutions can be identified to bring the signal from a terminal to a ground station. In the first solution, the signal is received by a satellite and immediately sent to a ground station in visibility. This solution ensures a real-time service but requires a very high number of ground stations to make sure that there is always a ground station in visibility. Another solution illustrated in FIG. 1A is to sample and store the signal received by the satellite in the on-board satellite memory. The stored samples are then sent to a ground station once there is one in visibility. Such a system requires a lower number of ground stations but cannot insure a real-time service. Furthermore, it requires a large memory on board to be able to store all the received samples and a high capacity feeder link to be able to send all the stored samples to the ground station. Yet another solution illustrated in FIG. 1B is to sample the signal received by the satellite and send the samples using Inter-Satellite-Link (hereafter ISL) to another satellite which will send the samples to a ground station. Such a system requires high capacity ISL and feeder link but can insure a real-time service. A fourth solution is to process (e.g. sampling and demodulating) the received signal on board before either send it to a ground station if there is one visible, or use ILS as described previously. It should be noted that since the signal is already processed the requirement in term of ILS or feeder link capacity is lowered compared to the solutions described previously. However, such a solution requires a high processing power on-board which also means a high-power consumption.
None of the above solutions propose a method where some messages can be processed so as to insure a real-time service while other messages can be processed so to insure an acceptable delay in the transmission.
There is a need of a method that allows a transmission system to insure, when needed, a short-delay delivery of messages sent by terminals, while limiting the impact of such a short delay delivery on the on-board processing power, on-board memory and on the bandwidth of the inter-satellite link. There is also a need for a method that takes advantage of high-gain antenna arrays, appropriate for communication with low power small terminals, even on-board small satellites with limited resources, obtaining a cost-effective link in terms of speed and cost per bit. There is also a need for a method that enables operation of small satellites in environments with potentially high interference from other uncorrelated terminals.